"Now" marks a break in the main story line. This new section gives background information about the men who served King David. (See: [[rc://en/ta/man/jit/writing-background]])
The phrase "was over" refers to having authority over a group of people. Alternate translation: "Joab had authority over ... Benaiah son of Jehoiada had authority over" (See: [[rc://en/ta/man/jit/figs-idiom]])